The invention described herein may be manufactured, used, and licensed by or for the United States Government for governmental purposes without payment to us of any royalties thereon.
The invention relates generally to electronic filters, and more particularly to a programmable SAW transversal filter utilizing a phase shifter.
In RF (radio frequency), electronic filters SAW (surface acoustic wave) devices are often used. It is often desirable to have RF electronic filters that have desirable electrical behavior, for example, low insertion loss, tunable bandwidth, tunable frequency, tunable filter transfer function. Additionally, it is desirable that such radio frequency electronic filters are capable of being fabricated as fully integrated components in integrated circuits. The radio frequency electronic filter ma y be used as a pre-selector or an IF (intermediate frequency) filter in a radio or communications receiver.
Transversal filters utilizing a SAW device are generally modeled as an input signal that passes through a series of delays. Subsequent to each delay, the signal is sampled or tapped. The various delayed and sampled or tapped signals are weighted in relation to a set of weighting coefficients. The output of the delayed, sampled and weighted signals is summed.
A surface acoustic wave device is comprised of a piezoelectric substrate upon which input and output interdigitated transducers (IDTs) are formed. The IDTs have fingers connected by busbars. The IDT launches an acoustic wave by the piezoelectric effect. The wave propagates as a surface acoustic wave to the output IDT. At each finger of the output IDT, the acoustic wave generates voltage as the wave travels underneath. Therefore, each finger voltage is delayed in time relative to the input by an amount dependent upon the acoustic wave velocity and the position of each finger of the interdigitated transducer. Weighting is often controlled by the length of overlap between adjacent fingers. The summing function is often obtained by busbars.
In order to permit a single device to perform a variety of functions, it is often desirable to provide a programmable SAW transverse filter. In a programmable SAW transverse filter, busbars are replaced by switches or other power splitter or combiner components. Additionally, a weighting function may be provided by gain elements permitting the fingers to have uniform physical overlap.
A programmable transversal filter is disclosed in U.S. Pat. No. 4,752,750 entitled xe2x80x9cHybrid Programmable Transversal Filterxe2x80x9d issuing to Zimmerman et al. on Jun. 21, 1988, which is herein incorporated by reference. Therein disclosed is a programmable transversal filter having a SAW delay line made of a first material and monolitholic dual gate FET (field effect transistor) arrays to control magnitude and sign of the tap weights made of a different material. Another transversal filter is disclosed in U.S. Pat. No. 5,051,709 entitled xe2x80x9cSAW Device Tapped Delay Line And Equalizerxe2x80x9d issuing to Birkett et al. on Sep. 24, 1991, which is herein incorporated by reference. Therein disclosed is a SAW device having tapped delay lines that includes apodized IDTs at the input and unapodized IDTs at the output. Each output IDT is terminated with a low impedance to minimize acoustic regeneration by the output IDTs. Grounded dummy fingers are used having the same metallization ratio as the output IDTs. Another transversal filter is disclosed in U.S. Pat. No. 5,194,837 entitled xe2x80x9cMulti-Tap Programming Circuit For Transversal Filtersxe2x80x9d issuing to Smythe, Jr. et al. on Mar. 16, 1993, which is herein incorporated by reference. Therein disclosed is a tapped delay line filter that has a varistor network for tap weighting. Yet another transversal filter is disclosed in U.S. Pat. No. 5,387,887 entitled xe2x80x9cMiniature Digitally Controlled Programmable Transverse Filter Using LSI GAAS Integrated Circuitsxe2x80x9d issuing to Zimmerman et al. on Feb. 7, 1995, which is herein incorporated by reference. Therein disclosed is a digitally controlled programmable transversal filter utilizing large scale integration gallium arsenide integrated circuits to digitally control the magnitude and sign of the tap weights.
While many of these prior devices address the programmability of transversal filters, problems still arise due to triple transient echo (TTE) in SAW programmable SAW transverse filters (PTFs). TTE arises from the basic nature of acoustic wave propagation in a SAW device. Each finger of the IDT is a source of reflection and regeneration of the acoustic wave. In the ideal case, only a single acoustic wave will travel from the input IDT to the output IDT. However, in most actual implementations of a SAW device an undesired signal is reflected from the output IDT back to the input IDT and then back to the output IDT. Each reflection includes a conversion loss due to the IDT bidirectionality, hence the TTE signal is an attenuated and delayed replica of the desired signal. Attempts to eliminate the undesirable TTE have taken the form of impedance mismatch and unidirectional IDT techniques. Impedance mismatch techniques are somewhat effective but cause high insertion loss. They are typically applied to IF filters and involve a system cost in terms of amplification required to make up for the insertion loss.
Several types of unidirectional transducers (UDT) exist. Most types rely on balancing electrode reflection and regeneration to transform the typically bidirectional IDT into a UDT. However, these devices have limitations. For example, all UDTs are inherently bandwidth limited. Additionally, one of the most significant limitations is that most UDTs require an asymmetric finger structure including fingers having varying widths at non-periodic spacing. This asymmetrical finger structure makes most UDT structures incompatible with programmable SAW filters. Therefore, there is a need for a programmable surface acoustic wave transversal filter that suppresses triple transient echo and yet provides low insertion loss.
The present invention comprises a multi-phase programmable transversal filter. A piezoelectric substrate has two regular arrays of electrode fingers formed thereon. At least some of the electrode fingers have a phase shift associated therewith. Each of the electrode fingers has a gain function associated therewith. A power splitter is associated with the input array to provide a signal to each of the input electrode fingers and a power combiner is associated with the output electrode fingers to provide a summed output. The electrode fingers in combination with the phase shifters result in a programmable multi phase unidirectional surface acoustic wave transversal filter.
Accordingly, it is an object of the present invention to suppress triple transient echo in a programmable transversal filter.
It is an advantage of the present invention that insertion loss is relatively low.
It is a feature of the present invention that unidirectional transducers are utilized.
It is yet another feature of the present invention that the unidirectional transducers are implemented in a multi phase arrangement and that a phase shifter is associated with at least some of the fingers of the unidirectional transducer.
These and other objects, advantages, and features will become readily apparent in view of the following detailed description.